The present invention concerns improvements in emissions control. More especially, the invention concerns improvements in the control of NOx from diesel engines.
The use of emission control catalysts for engine exhaust clean-up is well established. Diesel engines have different characteristics from gasoline-fuelled engines, with a different mix of pollutants caused by the different fuels, the different combustion characteristics in each engine and the lower temperatures met with in exhausts from diesel engines. Additionally diesel engines emit more noticeable particulates, especially under heavy load and upon start-up, than gasoline engines. In general, it can be said that diesel engines emit less NOx than a gasoline engine under most conditions, but because diesel engines mostly or exclusively operate on a high air to fuel ratio, that is are xe2x80x9cleanxe2x80x9d-bum engines, the chemistry of the exhaust gas does not favour NOx reduction, because of the excess of oxidising species.
To meet the various emission regulations already or about to enter force, it has become necessary to treat diesel exhausts in various ways. Oxidation catalysts, which catalyse the oxidation of unburnt hydrocarbons (xe2x80x9cHCsxe2x80x9d) and carbon monoxide (xe2x80x9cCOxe2x80x9d) are now regularly fitted to light duty diesels, and particulate traps of various types are becoming commonplace on heavy duty diesels as used in trucks, buses and some stationary engines. A technique for improving exhaust gas emissions, especially NOx emissions from diesel engines is exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d), which takes a proportion of the exhaust gas and recirculates it into the engine cylinders. Generally, about 30 to 60 vol % of the exhaust gases are recirculated, depending upon the characteristics of the particular engine and the emission limits which must be met. Although EGR has been used with gasoline engines for about ten years, it has only been more recently fitted to diesel engines; we believe that most vehicles currently fitted with EGR are passenger car light duty diesel engines. In the case of engines fitted with a catalyst, the exhaust gas is always taken from upstream of the catalyst. It is generally expected that EGR would have a significant effect on emissions from heavy duty diesel engines, that is those fitted to heavy trucks and buses. Because of the engineering problems caused by the very different exhaust characteristics compared to light duty diesel engines, however, this has proved difficult to achieve. In particular, there is no commercial source of an EGR valve of suitable size and materials to be fitted to a heavy duty diesel engine.
We refer also to a device marketed as the xe2x80x9cCRTxe2x80x9d by Johnson Matthey PLC. This device is described in U.S. Pat. No. 4,902,487 and is a continuously regenerative particulate trap. Unlike the vast majority of particulate traps, however, this device regenerates in situ without the need for periodic replacement or electrical heating to ignite the soot. Such device relies upon a catalyst system which generates NO2 which we found is effective to cause low temperature combustion of trapped soot particles.
The principle of the CRT has been adopted by Hino in their published Japanese patent applications JP 8338320 and JP 9088727, in combination with EGR. However, such systems as described are not believed to be capable of use in true heavy duty diesel applications.
We have surprisingly found that a novel diesel engine system can offer very low levels of NOx. The present invention provides a diesel engine system comprising a diesel engine and an exhaust system therefor, the exhaust system incorporating a catalyst effective to convert NO to NO2 under normal operating conditions, a trap for particulates mounted downstream of the catalyst and an exhaust gas recirculation valve with cooling means to cool the portion of exhaust gas which is recirculated, characterised in that the cooling means is mounted upstream of the exhaust gas recirculation valve.
The invention also provides a process for the reduction of NOx in diesel engine exhaust gases, comprising, in order, converting at least a portion of the NO in the gases to NO2 by passing the gases over a catalyst, trapping at least the majority of carbonaceous particles in the gases on a trap and continuously oxidising said particles and cooling and recirculating at least a portion of the cleaned gases leaving the trap, to the engine cylinders, characterised in that the gases are cooled before passing over an exhaust gas recirculation valve.
FIG. 1 shows a diesel engine incorporating an exhaust system of the present invention;
FIG. 2 shows NOx reduction in the system of the present invention.